Tag: motional emf

Questions Related to motional emf

Which of the following is true in context of Eddy currents?

eddy currents motional emf electromagnetic induction electromagnetic induction and alternating currents physics

  1. By Lenz's law, an eddy current creates a magnetic field that opposes the magnetic field that created it.

  2. Eddy currents are loops of electrical current induced within conductors by a changing magnetic field in the conductor, due to Faraday's law of induction

  3. Eddy currents are a cause of energy loss in alternating current (AC) inductors, transformers, electric motors and generators etc.

  4. All of the above 

Show answer
Correct Option: D
Explanation:

 Eddy currents are loops of electrical current induced within conductors by a changing magnetic field in the conductor, due to Faraday's law of induction,are a cause of energy loss in alternating current (AC) inductors, transformers, electric motors and generators etc.

By Lenz's law, an eddy current creates a magnetic field that opposes the magnetic field that created it.
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Eddy currents travel in

eddy currents motional emf electromagnetic induction electromagnetic induction and alternating currents physics

  1. closed paths.

  2. nonconductive materials.

  3. axially when testing rod with an encircling coil

  4. perpendicular to the test surface

Show answer
Correct Option: A
Explanation:

When there is a change in magnetic flux associated with a large piece of metal then currents are induced in that metal these currents are travelling in a closed circular path,  similar to Whirlpools or eddies in water. Such currents are known and Eddy Currents

Hysteresis loss and eddy current losses are used in

eddy currents motional emf electromagnetic induction electromagnetic induction and alternating currents physics

  1. induction heating of steel

  2. dielectric heating

  3. induction heating of brass

  4. none of the above

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Correct Option: A
Explanation:

There is a magnetic loss occur in ferromagnetic material like iron and steel in form of heat these losses are used for induction heating and cooking. For example $\rightarrow$ heating of transformer.

Which of the following is not an application of eddy currents.

eddy currents motional emf electromagnetic induction electromagnetic induction and alternating currents physics

  1. Induction heater

  2. Conductivity meters for non-magnetic metals

  3. Skin effect

  4. Coating Thickness Measurements

Show answer
Correct Option: C
Explanation:
Eddy currents are the currents induced in the body of a conductor when the amount of magnetic   flux linked with it changes
(i)    Induction furnace (heating effect of eddy current)
(ii)   Dead-beat galvanometer (electromagnetic damping)
(iii)  Electric brakes
(iv) Diathermy ( Strong 50 MHz A.C. passed through human body, also called deep heat treatment )
(v)  Induction motor
(vi) Car-speedometer
(vii)Energy meter

Silicon steel used in lamination mainly reduces

eddy currents motional emf electromagnetic induction electromagnetic induction and alternating currents physics

  1. Hysteresis loss

  2. Eddy current losses

  3. Copper loss

  4. All of the above 

Show answer
Correct Option: D
Explanation:

Silicon steel used in lamination mainly reducesHysteresis loss,Eddy current losses,Copper loss. In order to minimize the eddy currents more,magnetic core is laminated . The hysteresis coefficient of silicon steel is less, so hysteresis loss can be reduced. used mainly because of the following advantages: low hysteresis loss. low copper loss due to lower grain resistance.

In a given transformer for a given applied voltage, losses which remain constant irrespective of load changes are 

eddy currents motional emf electromagnetic induction electromagnetic induction and alternating currents physics

  1. friction and windage losses

  2. copper losses

  3. hysteresis and eddy current losses

  4. none of the above 

Show answer
Correct Option: C
Explanation:

In a given transformer for a given applied voltage, losses which remain constant irrespective of load changes are hysteresis and eddy current losses The losses that can occur in a material are: Iron losses: Iron loss is caused by the alternating flux in the core and consists of hysteresis and eddy current losses. of coercivity on the curve. (The reversed magnetizing force has flipped enough of the domains so that the net flux within the material is zero.)

Eddy currents are produced in a metallic conductor when

eddy currents motional emf electromagnetic induction electromagnetic induction and alternating currents physics

  1. The magnetic flux linked with it changes

  2. It is placed in a changing magnetic field.

  3. It is placed in a magnetic field.

  4. Both A and B

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Correct Option: D
Explanation:

Eddy currents are produced when the magnetic flux passing through the metal object continuously changes. This may happen due to many reasons:
1) The object is placed in a region with changing magnetic field.
2) The object continuously moves in and out of the magnetic field region (may be uniform or non uniform).

A magnet is dropped down an infinitely long vertical copper tube

eddy currents motional emf electromagnetic induction electromagnetic induction and alternating currents physics

  1. The magnet moves with continuously increasing velocity and ultimately acquires a constant terminal velocity

  2. The magnet moves with continuously decreasing velocity and ultimately comes to rest

  3. The magnet moves with continuously increasing velocity but constant acceleration

  4. The magnet moves with continuously increasing velocity and acceleration

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Correct Option: B
Which is the correct formula for calculating the power lost due to eddy currents per unit mass for a thin sheet or wire?? Where $P$ is the power lost per unit mass $(W/kg)$, $B _p$ is the peak magnetic field $(T)$, $d$ is the thickness of the sheet or diameter of the wire $(m)$, $f$ is the frequency $(Hz)$, $k$ is a constant equal to 1 for a thin sheet and 2 for a thin wire
eddy currents motional emf electromagnetic induction electromagnetic induction and alternating currents physics

  1. $P= \dfrac{{\pi}^2 {B _p}^2 d^2 f^2}{6k\rho D}$

  2. $P= \dfrac{{\pi}^2 {B _p}^2 d^2 f}{k\rho D}$

  3. $P= \dfrac{{\pi}^2 {B _p}^2 d^2 f^2}{6k\rho D^3}$

  4. $P= \dfrac{{\pi}^2 {B _p}^2 d^2 f}{6k\rho D^2}$

Show answer
Correct Option: A
Explanation:

(A)$P=\dfrac { { \pi  }^{ 2 }{ B } _{ p  }^{ 2 }{ d }^{ 2 }{ f }^{ 2 } }{ 6k\rho D } \ m{ L }^{ 2 }{ T }^{ -3 }=\dfrac { \left( M{ T }^{ -2 }{ A }^{ -1 } \right) ^{ 2 }\left( { L }^{ 2 } \right) \left( { T }^{ -2 } \right)  }{ M{ L }^{ -3 } } \ m{ L }^{ 2 }{ T }^{ -3 }=M{ L }^{ 2 }{ T }^{ -3 }\ m{ L }^{ 2 }{ T }^{ -3 }\neq m{ L }^{ 2 }{ T }^{ -2 }$ where A = amphere, T = Time, L = Length ,M = mass.

similarly C and D not matches so option A is correct .

A transformer core is laminated to 

eddy currents motional emf electromagnetic induction electromagnetic induction and alternating currents physics

  1. reduce hysteresis loss

  2. reduce eddy current loss

  3. reduce copper loss

  4. reduce all of the above loss

Show answer
Correct Option: B
Explanation:

The transformer core is laminated to break eddy currents to form loop and hence reducing the power loss due to heat generated due to eddy current.


Answer-(B)